<p><i>alias:</i> <b>ref-x</b></p>
<p>Make x-coordinate of the subelement relative to the x-coordinate of the element referenced to by the selector in <code>ref</code> attribute.
    If <code>ref-x</code> is in the <code>(0,1)</code> interval (or expressed in percentages, e.g. <code>'80%'</code>), the offset is
    calculated from the fraction of the bounding box of the referenced element. Otherwise, it is an absolute value in pixels.
</p>
